1. Field of the Invention
The present invention relates to a circuit for preventing unwanted oscillations in controlled power supply devices, and particularly to such a circuit for autoconverters.
2. Description of the Prior Art
Conventional controlled power supply devices tend toward oscillations. The reason for such oscillations is that the circuit portion effecting the control characteristically has, at least in certain operating regions, a negative slope. The negative slope represents the relationship between the input voltage and the input current. For example, if the output voltage with a given load resistor of a loss-free or low-loss autoconverter is to be controlled to a rated value, the load resistor absorbs a constant power, which is independent of the input voltage. The product of the input voltage and the input current of the controlled circuit portion is proportional to this power, that is, the product is also a constant. This has the consequence that the input current becomes lower with increasing voltage (and becomes higher with decreasing voltage), so that the differential input resistance of the controlled circuit portion is negative in certain operating regions. In that range of the input voltage in which the power absorbed by this circuit portion is essentially constant, the current-voltage relationship exhibits a hyperbolic path, so that the (negative) differential input resistance corresponding to the slope of this relationship is lowest when the input voltage is at a lowest value at which error-free control of the power supply device is still possible.
Conventional controlled power supply devices of this type have an input filter circuit for suppressing disruptive reactions on the feed network. This input circuit is generally comprised of at least one series inductance and at least one shunt capacitance. These two circuit elements, whose values are defined by the demands of an adequate filtering effect, represent an oscillatory structure which can be excited to independent oscillations in combination with the negative input resistance of the controlled circuit portion. Investigations of the stability conditions for this circuit show that oscillations can occur when the quotient of the value of the inductance and the product of the capacitance and the sum of the ohmic resistors involved (the internal resistance of the source and the equivalent series resistance of the capacitance) is greater than the amount of the (negative) input resistance of the controlled circuit portion. This condition applies approximately when the product of the ohmic resistances is low in comparison to the quotient of inductance and capacitance. Inversely, this means that stable operation is guaranteed when the sum of the ohmic resistances is greater than the quotient of the inductance and the product of the capacitance and the amount of the negative input impedance. For purpose of analysis, the negative input impedance is taken to be the minimum value possible in practical operation, which corresponds to the quotient of the lowest possible input voltage and the highest possible input current. Identifying this relationship would lead one to believe that the problem of undesirable oscillations can be eliminated by increasing the capacitance of the filter circuit and/or the ohmic resistance effective in series therewith. On the basis of such steps, the parallel circuit formed by the elements of the filter circuit is detuned and/or attenuated such that the stability condition is met. The increase of the effective capacitance may be achieved by the use of a high-capacitance electrolytic capacitor switched parallel to an existing high-quality pulse storage capacitor of the filtering circuit.
For modern power supply devices including extremely high output autoconverters, however, the above two approaches for eliminating the oscillatory tendency of the power supply are unsuitable. One of the most important advantages of such high-output autoconverters operated at a three-phase network is that the capacitance of their input circuit (the filter circuit) can be very low thus resulting in a correspondingly low network load. This advantage would be eliminated by increasing the capacitance for the purpose of solving the problem of oscillatory tendency.
Similarly, increasing the resistance which is effective in series with the capacitance is in theory possible for high-output autoconverters, but as a practical matter, results in an unacceptably high leakage loss through the resistor. This is particularly true for operation at a three-phase network wherein high superimposed a.c. components remain after rectification. This high dissipated power presents two problems, the first of dissipating the generated heat which arises in relatively small volumes, and the second being that the high power deteriorates the efficiency to an unacceptable degree, thus frustrating another important advantage of modern autoconverter circuit technology.